It has been proposed heretofore to use as a coating for air bearings a film of fluorinated carbon embedded in a polyimide of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane ("4-BDAF") and pyromellitic dianhydride ("PMDA"). In air bearings the rotating surfaces operate with a film of air between them and only occasionally do the metal surfaces come in actual contact with each other. Heretofore this coating composition has been indicated to be suitable for other, unspecified purposes.
It has also been proposed heretofore to utilize as a protective coating for piston skirts a system composed of graphite embedded in 4-BDAF/PMDA polyimide or in the polyimide of 4-BDAF and BTDA (benzophenonetetracarboxylic acid dianhydride). While this coating was reasonably effective (it survived for about 400 hours in engine dynamometer tests), a coating lasting at least 1,000 hours in this test is desired.